Direct production of graphite fibrous materials from preoxidized acrylic fibrous materials

ABSTRACT

A rapid process is provided for the direct conversion of a preoxidized acrylic fibrous material containing at least about 7 percent bound oxygen by weight to a fibrous material of predominantly graphitic carbon. The preoxidized acrylic fibrous material is initially impregnated with an organic protective agent and subsequently is passed through a reducing flame which imparts a minimum fiber temperature of at least 1,900*C. while the fibrous material is under tension at least sufficient to prevent visible sagging. In a preferred embodiment of the invention, the reducing flame is generated by a fuel-oxidant mixture, e.g., an acetylene and oxygen mixture.

United States Patent Stuetz [15] 3,656,903 [451 Apr. 18, 1972 [54]DIRECT PRODUCTION OF GRAPHITE FIBROUS MATERIALS FROM PREOXIDIZED ACRYLICFIBROUS MATERIALS [72] Inventor: Dagobert E. Stuetz, Westfield, NJ.

[73] Assignee: Celanese Corporation, New York, NY.

[22] Filed: Apr. 10, 1969 [21] Appl. No.: 815,200

s2 u.s.c1 ..23/209.1,23/209.2 [51] Int. Cl. ..C01b 31/07 [58] FieldofSearch ..23/209.1;8/115.5

[56] References Cited UNITED STATES PATENTS 3,281,261 10/1966 Lynch..23/209.1 X 3,285,696 11/1966 Tsunoda ..23/209.1 3,294,489 12/1966Millington et a] ..23/209.1 X 3,395,970 8/1968 Machell ..23/209.4 X3,412,062 11/1968 Johnson etal. ..260/37 3,427,120 2/1969 Shindo et a1...23/209.l X 3,449,077 6/ 1969 Stuetz ..23/209.l

OTHER PUBLICATIONS Vosburgh Textile Research Journal Vol. 30, 1960,pages 882- 896 Primary Examiner-Edward J. Meros Attorney-Thomas J.Morgan and C. B. Harris [57] ABSTRACT A rapid process is provided forthe direct conversion of a preoxidized acrylic fibrous materialcontaining at least about 7 percent bound oxygen by weight to a fibrousmaterial of predominantly graphitic carbon. The preoxidized acrylicfibrous material is initially impregnated with an organic protectiveagent and subsequently is passed through a reducing flame which impartsa minimum fiber temperature of at least 1,900C. while the fibrousmaterial is under tension at least sufficient to prevent visiblesagging. In a preferred embodiment of the invention, the reducing flameis generated by a fuel-oxidant mixture, e.g., an acetylene and oxygenmixture.

24 Claims, No Drawings DIRECT PRODUCTION OF GRAPHITE FIBROUS MATERIALSFROM PREOXIDIZED ACRYLIC FIBROUS MATERIALS BACKGROUND OF THE INVENTIONIn the search for high performance materials, considerable interest hasbeen focused on graphite fibers. Graphite fibers are defined herein asfibers which consist essentially of carbon and which have a predominantx-ray diffraction pattern characteristic of graphite. Amorphous carbonfibers, on the other hand, are defined as fibers in which the bulk ofthe fiber weight can be attributed to carbon and which exhibit anessentially amorphous x-ray diffraction pattern. Graphite fibersgenerally have a much higher modulus and a higher tenacity than doamorphous carbon fibers and in addition are more highly electrically andthermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, andgraphite fibers theoretically have among the best properties of anyfiber, including boron, for use as high strength reinforcement. Amongthese desirable properties are corrosion and high temperatureresistance, low density, high tensile strength, and most important, highmodulus. Graphite is one of the very few known materials whose tensilestrength increases with temperature. Uses for such graphite-reinforcedcomposites include aerospace structural components, rocket motorcasings, deep-submergence vessels and ablative materials for heatshields on reentry vehicles.

One of the major factors retarding the large-scale use ofgraphite-reinforced composites may be traced to the extreme costscommonly required for the production of high modulus graphite fiberssuitable for use as reinforcement. Although the production of fibrouscarbon by pyrolysis of hydrocarbon gases has been reported, thistechnique is generally not suitable for industrial applicationsrequiring good quality control. Graphitization of fibrous organicprecursors appears to be the only practical industrial route availableto form graphite fibers.

Many of the prior art methods for producing graphite fibers involve longprocessing periods, high power requirements, and/or expensive and bulkyheating apparatus, such as closed furnaces. The processing and equipmentcosts required to produce graphite fibers commonly dwarf the fiber rawmaterial costs. Often the fiber is of inferior quality due to damageoccurring in one or more steps used in its production. For example,amorphous carbon yarn can be converted to graphite yarn by furnacegraphitization in a high temperature vacuum oven. In such an oven thehot zone material has commonly been a 7 metal such as tungsten ortantalum. Besides being expensive and slow, this radiant heat methodalso may result in the deposition of foreign materials such as tungsten,tantalum and/or carbides of these metals on the fiber during the hightemperature treatment.

A'nother prior art approach to graphitizing amorphous car bon fiber,called conductive graphitization, involves passing a yarn overelectrically conductive contacts. For example, in one such method anamorphous carbon fiber is advanced over a pair of spaced electricallyconductive rollers while passing an electric current through theadvancing fiber to raise it to graphitization temperature. A controlledatmosphere of nitrogen, argon, carbon dioxide, or mixtures thereof,generally must be provided around the fiber while undergoing directresistance heating.

in co-pending U.S. Ser. No. 614,811, filed Feb. 9, 1967 in the names ofDagobert E. Stuetz, Leo R. Belohlav, and Arthur M. Reader, nowabandoned, a process is disclosed employing a reducing flame whereby apredominantly amorphous carbonaceous fibrous material containing atleast 75 per cent carbon by weight (preferably at least 90 per centcarbon by weight) is converted to a fibrous material of predominantlygraphitic carbon. In my co-pending U.S. Ser. No. 615,374, filed Feb. 13,1967, which issued as U.S. Pat. No. 3,449,077 on June 10, 1969, aprocess is disclosed whereby a polybenzimidazole fiber is initiallypreoxidized and subsequently is passed through a reducing flame where itis graphitized. The above-identified applications are assigned to thesame assignee as the instant invention. Heretofore when attempts havebeen made to directly graphitize a preoxidized acrylic fibrous materialby passage through a reducing flame as described hereafter the fibrousconfiguration has been destroyed. It has accordingly been deemedessential in the past to first carbonize the preoxidized acrylic fibrousmaterial to a carbon content of at least 75 per cent by weight prior topassage through the reducing flame in accordance with the teachings ofSer. No. 614,81 1.

It is an object of the invention to provide a rapid process for thedirect conversion of a preoxidized acrylic fibrous material to apredominantly graphitic fibrous material.

It is an object of the invention to provide a process capable ofproducing a graphite fiber exhibiting relatively uniform properties.

It is another object of the invention to provide a graphitizationprocess which does not require bulky and highly expensive equipment.

These and other objects, as well as the scope, nature and utilization ofthe invention will be apparent from the following detailed descriptionand appended claims.

SUMMARY OF THE INVENTION It has been found that a process for producinga predominantly graphitic fibrous material from a preoxidized acrylicfibrous material derived from a fibrous precursor consisting primarilyof recurring acrylonitrile units, with said preoxidized acrylic fibrousmaterial having a. a carbon content of up to about 65 per cent byweight,

b. a relatively amorphous x-ray diffraction pattern,

c. a bound oxygen content of at least about 7 per cent by weight, and

d. an. absence of structural integrity when heated for two seconds at afiber temperature of 1,900 C., comprises: impregnating the preoxidizedacrylic fibrous material with a quantity of an organic protective agentcapable of rendering the material resistant to loss of structuralintegrity when heated for at least two seconds at a fiber temperature ofl,900 C., and passing the impregnated fibrous material through areducing flame imparting to the material a minimum temperature of atleast l,900 C. at a speed sufficient to avoid breaking while thematerial is under a tension at least sufficient to prevent visiblesagging.

The resulting graphite fibers are suitable for use as a reinforcingmedium in composite articles which find particular utility inapplications where a strong, lightweight, structural element isrequired.

DETAILED DESCRIPTION OF THE INVENTION The preoxidized acrylic fibrousmaterial which is graphitized in accordance with the present process hasl) a carbon content of up to about 65 per cent by weight, (2) arelatively amorphous x-ray diffraction pattern, (3) a bound oxygencontent of at least 7 per cent by weight, and (4) an absence ofstructural integrity when heated for two seconds at a fiber temperatureof 1,900 C.

The carbon content of the preoxidized acrylic fibrous material will varywith the extent of the preoxidation treatment referred to hereafter, andwill commonly range from about 50 to 65 per cent by weight. As theextent of preoxidation increases, the carbon content of the preoxidizedfiber commonly tends to decrease. Since the preoxidation treatment isconducted at a moderate temperature, the preoxidized acrylic fibrousmaterial will exhibit a relatively amorphous xray diffraction patternand an absence of an x-ray diffraction pattern characteristic ofgraphitic carbon.

The bound oxygen content of the preoxidized acrylic fibrous material isat least about 7 per cent by weight as determinable by routineanalytical techniques, such as the Unterzaucher analysis. A preoxidizedacrylic fibrous material suitable for use in the process commonly has abound oxygen content of about 7 to per cent by weight. Preoxidizedacrylic fibrous materials having higher bound oxygen contents tend torequire more extended residence times for their production and tend toyield no commensurate advantage in the instant graphitization process.

In the absence of the impregnation treatment described hereafter thepreoxidized acrylic fibrous materials utilized in the process lose theirstructural integrity when heated for two seconds at afiber temperatureof l,900 C. produced by a reducing flame as described hereafter. Suchfailure of structural integrity is commonly exhibited by breakage of thefibrous material and partial or complete disintegration. The preoxidizedacrylic fibrous materials are commonly non-burning when subjected to anordinary match flame even in the absence of the impregnation treatment.

The acrylic fibrous precursor from which the preoxidized acrylic fibrousmaterial utilized in the process is derived consists primarily ofrecurring acrylonitrile units. For instance, the acrylic fibrousprecursor should generally contain at least about 85 mol per cent ofacrylonitrile units and up to about 15 mol per cent of one or moremonovinyl units copolymerized therewith, such as styrene, methylacrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidinechloride, vinyl pyridine, and the like. In a particularly preferredembodiment of the invention the preoxidized acrylic fibrous material isderived from an acrylonitrile homopolymer.

The preoxidized acrylic fibrous material is formed by the thermaltreatment of the fibrous precursor at a relatively moderate temperaturein an oxygen-containing atmosphere. The preoxidation treatment may beconducted on either a batch or a continuous basis in accordance withprocedures known in the art. US Ser. Nos. 749,957, filed Aug. 5, 1968 ofDagobert E. Stuetz; 749,959, filed Aug. 5, 1968 of Michael J. Ram; and750,018, filed Aug. 5, 1968 of Michael .1. Ram and Richard N. Rulisondisclose preferred preoxidation procedures. Each of the above-identifiedapplications is assigned to the same assignee as the instant inventionand is herein incorporated by reference. Other preoxidation procedurescapable of producing the requisite starting material for use in theinstant process may be chosen as will be apparent to those skilled inthe art.

The preoxidized acrylic fibrous materials which are graphitized inaccordance with the present invention are preferably in yarn form.Appreciable lengths of a continuous multifilament yarn are graphitizedin a particularly preferred embodiment of the invention. Staple yarnsmay also be selected, however, these generally give correspondinglylower tensile properties than do the continuous filament yarns. Otherfibrous assemblages, such as fabrics, may likewise be treated inaccordance with the present invention as will be apparent to thoseskilled in the art. When a yarn is selected as the starting material, itmay optionally be provided with a twist which improves its handlingcharacteristics. For example, a twist of about 0.1 to l tpi, andpreferably about 0.1 to 0.7 tpi may be conveniently utilized.

The preoxidized acrylic fibrous material is impregnated prior tosubjection to the reducing flame with an organic protective agent whichis capable of rendering the fibrous material resistant to loss ofstructural integrity when heated in a reducing flame for at least twoseconds at a fiber temperature of 1,900 C. As pointed out hereafter, theimpregnation step of the process may be conducted on either a batchbasis or on a continuous basis.

The organic protective agent may be selected from the group consistingof organic phosphorus compounds, organic antimony compounds, organic tincompounds, halogenated alcohols, halogenated esters, halogenated organicacids, halogenated organic anhydrides, halogenated phenolic compounds,halogenated aliphatic hydrocarbons, halogenated aromatic hydrocarbons,halogenated aromatic ethers, silicon oils, and mixtures of theforegoing.

A variety of organic phosphorus compounds may be selected for use in theprocess. For instance, organophosphorus compounds such as phosphates,phosphoramides, phosphites, phosphonates, etc. may be utilized. Examplesof particular organic phosphates which may be used aretris[2,3-dibromopropyl] phosphate available commercially under thedesignation Firemaster T23? from the Michigan Chemical Corporation,tris[beta-chloroethyl] phosphate, and tricresyl phosphate. Examples ofparticular phosphoramides which may be used are cyanoethyl bis[dimethylamino] phosphate and aziridinyl phosphine oxide. Examples of particularphosphites which may be used are triphenylphosphite, and condensationproducts of triphenylphosphite with bisphenol A (para,para'-isopropylidenediphenol) and pentaerythritol. Examples ofparticular phosphonates which may be used aremonochloromethylphosphonate, and dimethyl[trichloro alpha-hydroxyethyl]phosphonate. Phosphite-phosphonates sold under the designation ofPhosgard by the Monsanto Company may be selected. Diphosphonium halidessuch as ethylene bis[tricyanoethyl] phosphonium bromide,tetrakis[hydroxymethyl] phosphonium chloride, andtetrakis[hydroxymethyl] phosphonium hydroxide are also representative oforganophosphorus compounds which are suitable for use.

Examples of particular organic antimony compounds which may be used aretriphenylantimony (triphenylstibine), diphenylantimony,trichloroantimony, tribromoantimony, and antimony salts of carboxylicacids, such as antimony tricaproate.

Suitable organic tin compounds include tin salts of carboxylic acids,e.g. tin octoate, tin oleate, and the various tin acrylates.

Suitable halogenated alcohols include bis[2,3- dibromopropyl]pentaerythritol, the polychlorobutanediols, and 2,3-dibromopropanol.

Examples of particular halogenated esters which may be used areDiels-Alder adducts (essentially equimolar) of alphabeta unsaturatedcarboxylic acids (e.g. acrylic, methacrylic, and crotonic acid) withhexachlorocyclopentadiene.

Exemplary halogenated organic acids are chlorendic acid(hexachloroendomethylenetetrahydrophthalic acid) which may beconveniently fonned by the hydrolysis of chlorendic anhydride, andtetrabromophthalic acid.

Examples of halogenated organic anhydrides which may be used arechlorendic anhydride (i.e. the Diels-Alder adduct of maleic anhydridewith hexachlorocyclopentadiene), and tetrabromophthalic anhydrideavailable commercially under the designation Firemaster PHT4 from theMichigan Chemical Corporation.

Suitable halogenated phenolic compounds include pentachlorophenol,pentabromophenol, and halogenated compounds of bisphenol A (i.e. para,para-isopropylidenediphenol), such as tetrabromobisphenol A (i.e.4,4'-ispropylidenebis[2,6-dibromophenol]) available commercially underthe designation Firemaster BP4A" from the Michigan Chemical Corporation.

Examples of halogenated aliphatic hydrocarbons are the chlorinatedhigher paraffins, e.g. chlorinated paraffins having at least six carbonatoms per molecule. Such chlorinated hydrocarbons commonly contain about10 to 30 or more carbon atoms per molecule and are mainly saturatedstraight chain hydrocarbons which range from viscous liquids to solidsat room temperature. Such chlorinated paraffms are well known in the artand may contain various degrees of chlorination which generally range upto about 70 per cent chlorine by weight, e.g. about 40 to about 70 percent chlorine by weight. Chlorinated paraffins are availablecommercially under the designation Unichlor" (e.g. Unichlor 40,"Unichlor 50," and Unichlor 70) from the Neville Chemical Company, andChlorowax from the Diamond Alkali Co. Additionally, suitable halogenatedaliphatic hydrocarbons may process a cyclic structure and includeperchloropentacyclodecane, hexabromocyclododecane andhexabromododecatriene.

Suitable halogenated aromatic hydrocarbons include pentabromobenzene,pentabromotoluene, and pentachlorotoluene.

Exemplary halogenated aromatic ethers are pentabromodiphenylether,pentachlorodiphenylether, and dibromophenylether.

The silicone oils contemplated for use in the process of the presentinvention are water-insoluble, substantially nonvolatile, liquidpolysiloxanes, and any of the commonly known compositions of this typemay be employed. Polysiloxanes wherein the organic radical is a lowmolecular weight aliphatic group such as methyl or ethyl, or wherein ahigh percentage of low molecular weight aliphatic groups are present,are the types most widely available and have the lowest cost, and arefor this reason preferred. Examples of particular silicone oils whichmay be used are the polymethyl s iloxanes having a viscosity of over 50centistokes at 25 C. (Dow Corning DC- 200 series) and polymethyl phenylsiloxanes of medium aromaticity (Dow Corning DC-550 silicone oil).Another commercially available material is known as General ElectricSilicone Oil No. 9981. The silicone oils are well understood in the art,being described, for instance, in chapter 4 of Chemistry of theSilicones" by Eugene G. Rochow, 1st. Ed., 1946, and further described inchapters 5 and 6 of the 2nd edition of this book, published in 1951. Thealkyl, aryl, and alkyl-aryl silicones mentioned as oils in thesechapters are suitable for use according to the present invention.

Mixtures of the foregoing organic protective agents may be utilized toimpregnate the preoxidized acrylic fibrous material. One may alsooptionally employ an additive in conjunction with the protective agentwhich enhances the operation of the protective agent. For example,bibenzyl (l,2-diphenylethane) has been found to exhibit a synergisticinfluence.

The mechanism whereby the organic protective agent renders thepreoxidized acrylic fibrous material capable of withstanding the highlyelevated temperature of the reducing flame (described in detailhereafter) is considered complex and incapable of simple explanation.The result obtained is considered surprising in view of the fact thatthe organic compounds utilized are themselves not generally recognizedto be capable of enduring the temperature of the reducing flame. Forinstance, it is difficult to envision how the organic protective agentsperform their function since these agents are observed substantially tovolatilize at the high temperature of the reducing flame. While some ofthe organic protective agents have been utilized as flame retardants inthe past they have been generally considered to have no flame retardantactivity above about 500 C. Conversely, well known inorganic flameretardants, such as boric acid, which have in the past been suggested'as high temperature flame retardants do not provide the protectionrequired for successful graphitization of a preoxidized acrylic fibrousmaterial in the reducing flame.

if the organic protective agent is a non-viscous liquid, the fibrousmaterial may be simply contacted with the same. Most commonly however,the organic protective agent is a viscous liquid or solid. When theorganic protective agent is a viscous liquid or solid, it is dissolvedin a suitable solvent or carrier in order to facilitate impregnation ofthe fibrous material. The specific solvent selected is dictated by thesolubility characteristics of the organic protective agent as will beapparent to those skilled in the art and is relatively volatile. Forsafety considerations it is recommended that solvents be employed whichpossess a relatively high self-ignition point, and which are accordinglynot highly flammable.

In a preferred embodiment of the process the preoxidized acrylic fibrousmaterial is impregnated by immersion in a liquid comprising the organicprotective agent. For instance, the fibrous material while present on amandrel, bobbin, or other support may be placed in the liquid, or acontinuous length of the material may be continuously passed through theliquid. Alternatively, the liquid may be sprayed or padded upon thefibrous material; however, superior impregnation is generally achievedthrough immersion and is accordingly recommended. The temperature of theliquid at the time of impregnation is not critical and may convenientlybe room temperature. A highly volatile non-viscous halogenatedhydrocarbon solvent may be selected to dissolve many of the organicprotective agents. For instance, highly halogenated hydrocarbons such ascarbon tetrachloride, perchloroethylene, tetrachloroethane,tetrabromoethane, dibromoethane, and trichloroethylene are preferred. Aparticularly preferred solvent is trichloroethylene. Organic protectiveagents such as tetrakis[ [hydroxymethyl] phosphonium chloride may beapplied from water solutions. The concentration of the organicprotective agent in a solvent required to achieve adequate loading ofthe fiber within a brief period of immersion (e.g. 5 seconds) is nothighly critical. For instance, concentrations of the organic protectiveagents in solvents generally may conveniently range from about 0.5 to 25per cent by weight based upon the weight of the solvent (preferablyabout 1 to 10 per cent by weight based upon the weight of the solvent).Higher concentrations tend to yield no commensurate advantage.

The residence time during which the preoxidized acrylic fibrous materialis immersed in a solution comprising the preoxidized acrylic fibrousmaterial is somewhat dependent upon the form assumed by the materialwhile immersed. If the fibrous material is present as a package, alonger residence time will be required for the material to beimpregnated throughout. For instance, residence times in batchoperations for the impregnation step may range as high as 5 minutes ormore depending upon the thickness of the package. In a continuousoperation in which a continuous length of the preoxidized acrylicfibrous material is continuously passed through the solution comprisingthe organic protective agent, the residence time conveniently may befrom about 5 to 25 seconds, since adequate impregnation and coating ofthe fibrous material is not dependent upon extended soaking.

It is not essential that the preoxidized acrylic fibrous material bedried prior to its passage through the reducing flame. In fact, in apreferred embodiment of the process the fibrous material is continuouslypassed through the impregnation zone and then directly to the reducingflame for graphitization.

A highly preferred reducing flame for use in the process is thatresulting from an acetylene and oxygen mixture. With this reducingflame, the graphitizing step can be conducted in an open atmosphere. Afurther advantage of the acetylene-oxygen reducing flame is that it hasa fairly constant high temperature which is independent, within limits,of the fuel-oxidant ratio. A carbon monoxide-oxygen flame also yieldsgood results in an open atmosphere, although it is, of course, essentialto provide adequate safety provisions for the operator under suchconditions. Hydrocarbon fuels, such as propane and butane, may beselected but the process does not proceed as smoothly as with acetyleneor with carbon monoxide. However, in the presence of an inert blanketinggas, e.g. nitrogen, or argon, comparable stability may be achieved withhydrocarbon fuels.

Molecular oxygen can be replaced in the fuel-oxidant mixture by agaseous oxidant such as nitrous oxide although generally it is notadvantageous to do so because of the convenience and ready availabilityof oxygen. Fuel-oxidant combinations can also be employed to produce thereducing flame which do not contain a hydrocarbon fuel, such as a carbonmonoxide-hydrogen mixture and a hydrogen-chlorine mixture.Non-conventional flame sources, such as augmented flames (cf. B.Karlovitz, International Science of Technology, June, 1962, pp. 36-41);recombination flames, such as the atomic hydrogen torch (cf. 1.Langmuir, Industrial and Engineering Chemistry, June, 1927, pp.667-674); plasma torches; and the like; can also be employed to providehigh temperatures. The temperature should not be so high, however, as todestroy the fibrous configuration.

The fuel to oxidant ratio generally is a significant parameter in thepresent process. The graphitizing treatment is best carried out in aluminous flame obtained by keeping the amount of oxygen in the fuelmixture below the stoichiometric amount which is required to burn thefuel completely to carbon dioxide. Oxidation reactions within the flameare essentially limited to the combustible gas mixture, and the fibrousmaterial traveling through the flame is exposed to an essentiallyreducing environment. The fibrous material can be destroyed by oxidationif the oxidant-fuel ratio is too high. The luminosity of the flame isbelieved to be caused by ionized carbon fragments in the flame resultingfrom incomplete combustion of the fuel. More pyrolytic carbon will bedeposited at higher oxygen to fuel ratios than at lower ratios. Forcertain applications a deposit of pyrolytic graphite is desirable sinceit increases the high temperature stability of the resulting graphiticproduct. For other applications such a deposit is undesirable, e.g.,where high adhesion to a matrix is desired. Hence, the flexibility ofprocessing conditions allows for the production of a variety of graphiteproduct types. If desired, a surface protective layer of pyrolyticgraphite alternatively can be formed in a separate step in which thefibrous material is heated to a high temperature in a controlledenvironment containing hydrocarbon vapors.

In the context of this specification, temperatures in the flame zonerefer to the temperature of the fibrous material as measured by aninfra-red radiation thermometer and not to a theoretical reducing flametemperature under adiabatic conditions, i.e. without withdrawal of heatby immersing a body into the reducing flame. The temperature of thefibrous material in the flame is generally significantly lower than thetheoretical reducing flame temperature.

For example, the theoretical flame temperature of an oxygen-acetylenereducing flame is about 3,lO C. An upper limit of about 2,500 C. for thefiber temperature of a yarn undergoing treatment is generally sufficientand safe.

The impregnated fibrous material is passed through the reducing flame ata fast enough rate to avoid breaking. As the temperature of the reducingflame is increased, the minimum rate at which breakage is avoided alsoincreases. This minimum speed can be determined for any givencombination of impregnated fibrous material and specific reducing flame.The longer the residence time, the greater the extent of graphitization.Thermal degradation with graphite formation occurs within the reducingflame to the substantial exclusion of oxidative degradation. Optimumconditions are reached at the point where the loss of the fiber mass islowest and the conversion of the remainder of the fibrous material tographitic carbon is the highest. The two effects can be balancedfavorably by adjusting the residence time and the fiber temperature.Subject to the nature of the reducing flame and other factors, residencetimes of about 2 to 24 seconds, and preferably about 6 to 17 seconds aregenerally suitable. An exemplary set of optimum conditions is a yarntemperature of 2,300 C. with a residence time of about seconds.

The necessary apparatus for graphitization according to the presentprocess is simple and should be so arranged that the fibrous material isexposed to a minimum of frictional contacts. A convenient arrangement isto continuously feed an impregnated yarn from a rotating-reciprocatingbobbin through the reducing flame to an identically functioning takeupmechanism. Starting at the correct reciprocating position on the bobbin,the yarn is unwound without traverse movement and analogously rewound atthe take-up side. Hence, random yarn motions are minimized. Furtherpositioning of the yarn in the reducing flame can be accomplished withminimal action by two cylindrically shaped guides located before andafter the reducing flame source. Feed and take-up bobbins can be drivenby, for example, solid-state controlled DC motors with r.p.m.generator/indicators. When an inert atmosphere is desired, a cruciformglass vessel fitted with cooling plates and a passage opening for theyarn can be employed. Before entering the burner module, the yarn may beput under constant tension as, for example, by passing it over arubber-capped electro-magnetic clutch and a skewed roll. The

latter separates individual yarn loops around the clutch and preventsabrasion by yarn to yarn contact.

The geometry of the burner is also a factor in maximizing theeffectiveness of the graphitization of the present invention. Twoimpinging flames originating from two standard conical tipssignificantly raise the temperature of the fibrous material passingtherethrough. A particularly preferred embodiment of this technique isthe impingement of the two reducing flames on the tips of their innercones at an angle of 45. However, the addition of more than two orificesdoes not have a beneficial effect. A series of burners such as to form acontinuous reducing flame zone of increased lateral dimension may permitincreased processing speeds or the processing of larger fibrousassemblages. Since residence time in the reducing flame is a majorparameter, processing speed is significantly related to the length ofthe flame zone. Surrounding the conical tip with a cylindrical orglobular reflector, constructed from polished stainless steel sheets,for example, also significantly raises the yarn temperature.

The volume flow of the fuel and oxidant through the burner should be ashigh as possible, consistant with good reducing flame stability, inorder to maximize the moduli of the fibers.

Tension during reducing flame treatment is recommended for theachievement of optimum properties as it prevents the tendency of thefibrous material to shrink. Shrinkage usually leads to relaxation ofordered structures and, thereby, causes a lowering of physicalproperties. Preservation of orientation and/or an increase inorientation, depending upon the magnitude of tension applied, increasesboth the Youngs modulus and the tensile strength. The tension appliedshould be at least sufficient to avoid visible sagging. Beyond theoptimum tension the fibrous material may be damaged by still highertensions.

Graphite fibers produced according to the present process may possessrelatively uniform properties. Although i do not wish to be limited bythe theory of this improvement, it appears to be due to two factors: (1)the flame healing of flaws by ionized carbon fragments in a luminousreducing flame, and (2) the reduced mechanical wear of the fibrousmaterial in the reducing flame.

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific embodiments which follow.

EXAMPLE 1 The data in Table I illustrate the results obtainable to anembodiment of the invention wherein a tris[2,3-dibromopropyl] phosphateorganic protective agent is applied on a batch basis and anacetylene-oxygen reducing flame source is utilized to form a graphiteyarn from a preoxidized acrylonitrile homopolymer yarn.

A 760 continuous filament acrylonitrile homopolymer yarn was preoxidizedin air at about 270 C. in a Lindberg muffle fumace on a continuous basisin accordance with the teachings of US. Ser. No. 749,957, filed on Aug.5, 1968 in my name, which is assigned to the same assignee as theinstant invention, and is herein incorporated by reference. Theacrylonitrile homopolymer yarn had a twist of about 0.5 tpi. Thepreoxidized yarn exhibited a bound oxygen content of about 8 per cent byweight as determined by the Unterzaucher analysis and a carbon contentof about 62 per cent by weight. The preoxidized yarn was non-buming whensubjected to an ordinary match flame, but was incapable of withstandinga reducing flame when heated for two seconds at a fiber temperature ofl,900 C. The preoxidized yarn exhibited an amorphous X-ray diffractionpattern, and had a denier of 1.93, a single filament tenacity of 1.89grams/denier, a single filament Youngs modulus of about grams/denier,and an elongation of 7.3 per cent.

A 10 per cent by weight solution of tris[2,3-dibromopropyl phosphate intrichloroethylene based upon the weight of the solvent was prepared andpresent at room temperature (i.e. about 23 C.). The preoxidizedacrylonitrile homopolymer yarn while wound on a bobbin was immersed inthe solution for about 1 Va minutes in order to thoroughly impregnatethe same with he organic protective agent.

The yarn was next continuously graphitized without drying the same. Inthe manner described above the yarn was passed in the open atmospherethrough an acetylene-oxygen reducing flame in which the volume ratio ofthe gases was 1:1. The total flow rate was 1,500 mL/min. and a NationalBlow Torch", Tip A-3 was employed. The acetylene-oxygen reducing flameimparted a fiber temperature of about 2,200 C. to the yarn. The tensionapplied to the yarn was varied as indicated and no visible sagging ofthe yarn was apparent in any of the runs. The following Table Isummarizes the properties (single filament breaks) of the resultinggraphite yarn as a function of the residence time of the yarn in thereducing flame.

TABLE I Young's Residence time Tenacity modulus in reducing flameTension (grams/ (grams/ (seconds) (grams) Denier denier) denier) EXAMPLE2 The data in Table II illustrate the results obtainable in anembodiment of the invention wherein a tris[2,3- dibromopropyl] phosphateprotective agent is applied on a continuous basis to the preoxidizedacrylic fibrous material from solutions of the agent of variousconcentrations in trichloroethylene.

The preoxidized acrylic fibrous material identified in Example l wascontinuously passed through the solutions containing the organicprotective agent for a residence time of about 8 seconds, andsubsequently flame graphitized according to the procedure of Example 1without first drying the same. In each instance the tension applied tothe yarn while present in the reducing flame was sufficient to preventvisible sagging. The following Table II summarizes the properties(single filament breaks) of the resulting graphite yarn as a function ofthe concentration of the protective agent in the trichloroethylenesolvent in weight per cent based upon the weight of the solvent.

ill Residence time in reducing flame (seconds)- 6 Tension 0, Denier 0.64 .Tenacity (grams/denier) 4. 2 Youngs modulus (grams/denier) l, 383

EXAMPLE 4 Example 2 was repeated with the exception that the organicprotective agent was solely the silicone oil described in Example 3which was applied from a 10 per cent by weight solution intrichloroethylene based on the weight of the solvent.

Summarized below are the graphitization conditions and the properties(single filament breaks) of the resulting graphite yarn.

Residence time in reducing flame (seconds) 8 Tension 0 Denier 0. 70Tenacity (grarns/ denier) 1. 75 Youngs modulus (grams/denier) 852EXAMPLE 5 Residence time in reducing flame (seconds)- 6 Tension 0 Denier0. 72 Tenacity (grams/denier) 3. 45

Xge s eieq e eesldesisxl; l

EXAMPLE 6 Example 2 was repeated with the exception that the organicprotective agent comprised a combination l per cent by weight of thesilicone oil described in Example 3, and l per cent by weight ofbibenzyl in trichloroethylene based upon the weight of the solvent.

Summarized below are the graphitization conditions and the properties(single filament breaks) of the resulting graphite yarn.

TABLE II Residence time in Young's Concentration of protective reducingTenacity modulus agent in sol vent flame Tension (grams! (grams/(percent) (seconds) (grams) Denier denier) denier) 7.5 8 0 0.71 5.51,125 '3 0 0.86 5.0 1,303 8 0 0. 84 5.1 1,107

Residence time in reducing flame (seconds) 6 EXAMPLE 3 Tension 0 Denier0. 62 Example 2 was repeated with the exception that the T it(grams/denier) 3. 0 trichloroethylene solution of orgamc protectiveagent com- Youngs modulus (grarnsLdeniel') 1 9% prised a combination of10 per cent tris[2,3-dibromopropyl] phosphate, 1 per cent bibenzyl, andl per cent silicone oil, and the preoxidized yarn was present in thesame for a residence time of about 6 seconds. The concentrations areexpressed in weight per cent based on the weight of the solvent. Thesilicone oil was a polymethyl siloxane having a viscosity in excess of50 centistokes at 25 C. and available commercially from Elbe) [30wCorning Corporation under the designation DC Summarized below are thegraphitization conditions and the properties (single filament breaks) ofthe resulting graphite yarn.

EXAMPLE 7 and passage openings for the fiber. A surface-mixpropane-oxygen burner is mounted within the vessel.

EXAMPLE 8 Example 2 is repeated with the exception that the organicprotective agent is present as a 7.5 per cent by weight solution basedupon the weight of the solvent of triphenylantimony intrichloroethylene.

EXAMPLE 9 Example 2 is repeated with the exception that the organicprotective agent is present as a per cent by weight solution based uponthe weight of the solvent of antimony tricaproate in trichloroethylene.

EXAMPLE 10 Example 2 is repeated with the exception that the organicprotective agent is present as a 7.5 per cent by weight solution basedupon the weight of the solvent of tin octoate in trichloroethylene.

EXAMPLE I 1 Example 2 is repeated with the exception that the organicprotective agent is present as a 10 per cent by weight solution basedupon the weight of the solvent of bis[2,3- dibromopropyl]pentaerythritol in trichloroethylene.

EXAMPLE12 Example 2 is repeated with the exception that the organicprotective agent is present as a 10 per cent by weight solution basedupon the weight of the solvent of an equimolar Diels- Alder adduct ofmethacrylic acid with hexachlorocyclopentadiene in whichtrichloroethylene serves as solvent.

EXAMPLE 13 Example 2 is repeated with the exception that the organicprotective agent is present as a 5 per cent by weight solution basedupon the weight of the solvent of chlorendic acid(hexachloroendomethylenetetrahydrophthalic acid) in trichloroethylene.

EXAMPLE 14 Example 2 is repeated with the exception that the organicprotective agent is a 7.5 per cent by weight solution based upon theweight of the solvent of tetrabromophthalic anhydride intrichloroethylene.

EXAMPLE 15 Example 2 is repeated with the exception that the organicprotective agent is a 10 per cent by weight solution based upon theweight of the solvent of pentachlorophenol in trichloroethylene.

EXAMPLE 16 EXAMPLE 17 Example 2 is repeated with the exception that theorganic protective agent is a 5 per cent by weight solution based uponthe weight of the solvent of hexabromocyclododecane intrichloroethylene.

EXAMPLE 18 Example 2 is repeated with the exception that the organicprotective agent is a 5 per cent by weight solution based upon theweight of the solvent of pentabromotoluene in trichloroethylene.

In a control experiment in which the preoxidized acrylonitrilehomopolymer yarn was impregnated with a 4 per cent by weight aqueoussolution of boric acid based upon the weight of the solvent the yarnimmediately broke upon subjection of the reducing flame.

Although the invention has been described with preferred embodiments itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and scope of theclaims appended hereto.

lclaim:

1. A process for producing a predominantly graphitic fibrous materialfrom a preoxidized acrylic fibrous material derived from a fibrousprecursor consisting primarily of recurring acrylonitrile units, saidpreoxidized acrylic fibrous material having a. a carbon content of up toabout 65 per cent by weight,

b. a relatively amorphous X-ray diffraction pattern,

c. a bound oxygen content of at least about 7 per cent by weight, and

d. an absence of structural integrity when heated for two seconds at afiber temperature of 1,900 C., comprising:

impregnating said preoxidized acrylic fibrous material with a quantityof an organic protective agent capable of rendering said materialresistant to loss of structural integrity when heated for at least twoseconds at a fiber temperature of l,900 C., and

passing said impregnated fibrous material through a reducing flameimparting to said material a minimum temperature of at least 1,900 C. ata speed sufficient to avoid breaking while said material is under atension at least sufficient to prevent visible sagging.

2. A process according to claim 1 wherein said preoxidized acrylicfibrous material is derived from an acrylonitrile homopolymer.

3. A process according to claim 1 wherein said preoxidized acrylicfibrous material is derived from an acrylonitrile copolymer whichcontains at least about mol per cent of acrylonitrile units and up toabout 15 mol per cent of one or more monovinyl units copolymerizedtherewith.

4. A process according to claim 1 wherein said preoxidized acrylicfibrous material is impregnated with said organic protective agent byimmersion in the same.

5. A process according to claim 4 wherein said preoxidized acrylicfibrous material is immersed in a solvent containing said organicprotective agent dissolved therein.

6. A process according to claim 1 wherein said organic protective agentis selected from the group consisting of organic phosphorus compounds,organic antimony compounds, organic tin compounds, halogenated alcohols,halogenated esters, halogenated organic acids, halogenated organicanhydrides, halogenated phenolic compounds, halogenated aliphatichydrocarbons, halogenated aromatic hydrocarbons, halogenated aromaticethers, silicone oils, and mixtures of the foregoing.

7. A process according to claim 1 wherein said reducing flame isgenerated by a fuel-oxidant mixture.

8. A process according to claim 7 wherein said oxidant is oxygen.

9. A process according to claim 8 wherein said fuel is acetylene.

10. A process according to claim 8 wherein said fuel is propane.

11. A process according to claim 7 wherein said impregnated fibrousmaterial is passed through the luminous portion of said reducing flame.

12. A process according to claim 7 wherein the residence time of saidimpregnated fibrous material in said reducing flame is from about 2 to24 seconds.

13. A process according to claim 9 wherein the residence time of saidimpregnated fibrous material in said reducing flame is from about 6 to17 seconds.

14. A process according to claim 8 wherein the ratio of oxygen to fuelis such that the amount of oxygen is less than the stoichiometric amountrequired to completely oxidize said fuel.

15. A process according to claim wherein an inert atmosphere is providedaround said reducing flame.

16. A process according to claim 1 wherein said preoxidized acrylicfibrous material is a yarn.

17. A continuous process for producing a predominantly graphitic fibrousmaterial from a preoxidized acrylic fibrous material derived from afibrous precursor consisting primarily of recurring acrylonitrile units,said preoxidized acrylic fibrous material having a. a carbon content ofup to about 65 per cent by weight,

b. a relatively amorphous X-ray diffraction pattern,

c. a bound oxygen content of at least about 7 per cent by weight, and

d. an absence of structural integrity when heated for two seconds at afiber temperature of l,900 C., comprising:

passing said preoxidized acrylic fibrous material through a zonecontaining an organic protective agent capable of rendering saidmaterial resistant to loss of structural integrity when heated for atleast two seconds at a fiber temperature of l,900 C. selected from thegroup consisting of organic phosphorus compounds, organic antimonycompounds, organic tin compounds, halogenated alcohols, halogenatedesters, halogenated organic acids, halogenated organic anhydrides,halogenated phenolic compounds, halogenated aliphatic hydrocarbons,halogenated aromatic hydrocarbons, halogenated aromatic ethers, siliconeoils, and mixtures of the foregoing, wherein said preoxidized acrylicfibrous material is impregnated with said organic protective agent, and

passing said impregnated fibrous material through the luminous portionof a reducing flame generated by an acetylene-oxygen mixture impartingto said fibrous material a minimum temperature of at least l,900 C. inwhich the ratio of oxygen to acetylene is less than the stoichiometricamount required to completely oxidize the acetylene for a residence timeof about 2 to 24 seconds while said fibrous material is under a tensionat least sufficient to prevent visible sagging.

18. A continuous process according to claim 17 wherein said preoxidizedacrylic fibrous material is derived from an acrylonitrile homopolymer.

19. A continuous process according to claim 17 wherein said preoxidizedacrylic fibrous material is derived from an acrylonitrile copolymerwhich contains at least about mol per cent of acrylonitrile units and upto about l5 mol per cent of one or more monovinyl units copolymerizedtherewith.

20. A continuous process according to claim 17 wherein said preoxidizedacrylic fibrous material is passed through a zone containing saidorganic protective agent dissolved in a solvent.

21. A continuous process according to claim 20 wherein said solvent is ahalogenated hydrocarbon.

22. A continuous process according to claim 21 wherein said solvent istrichloroethylene.

23. A continuous process according to claim 17 wherein the residence forsaid impregnated fibrous material in said reducing flame is about 6 to17 seconds.

24. A continuous process according to claim 17 wherein said preoxidizedacrylic fibrous material is a yarn.

2. A process according to claim 1 wherein said preoxidized acrylicfibrous material is derived from an acrylonitrile homopolymer.
 3. Aprocess according to claim 1 wherein said preoxidized acrylic fibrousmaterial is derived from an acrylonitrile copolymer which contains atleast about 85 mol per cent of acrylonitrile units and up to about 15mol per cent of one or more monovinyl units copolymerized therewith. 4.A process according to claim 1 wherein said preoxidized acrylic fibrousmaterial is impregnated with said organic protective agent by immersionin the same.
 5. A process according to claim 4 wherein said preoxidizedacrylic fibrous material is immersed in a solvent containing saidorganic protective agent dissolved therein.
 6. A process according toclaim 1 wherein said organic protective agent is selected from the groupconsisting of organic phosphorus compounds, organic antimony compounds,organic tin compounds, halogenated alcohols, halogenated esters,halogenated organic acids, halogenated organic anhydrides, halogenatedphenolic compounds, halogenated aliphatic hydrocarbons, halogenatedaromatic hydrocarbons, halogenated aromatic ethers, silicone oils, andmixtures of the foregoing.
 7. A process according to claim 1 whereinsaid reducing flame is generated by a fuel-oxidant mixture.
 8. A processaccording to claim 7 wherein said oxidant is oxygen.
 9. A processaccording to claim 8 wherein said fuel is acetylene.
 10. A processaccording to claim 8 wherein said fuel is propane.
 11. A processaccording to claim 7 wherein said impregnated fibrous material is passedthrough the luminous portion of said reducing flame.
 12. A processaccording to claim 7 wherein the residence time of said impregnatedfibrous material in said reducing flame is from about 2 to 24 seconds.13. A process according to claim 9 wherein the residence time of saidimpregnated fibrous material in said reducing flame is from about 6 to17 seconds.
 14. A process according to claim 8 wherein the ratio ofoxygen to fuel is such that the amount of oxygen is less than thestoichiometric amount required to completely oxidize said fuel.
 15. Aprocess according to claim 10 wherein an inert atmosphere is providedaround said reducing flame.
 16. A process according to claim 1 whereinsaid preoxidized acrylic fibrous material is a yarn.
 17. A continuousprocess for producing a predominantly graphitic fibrous material from apreoxidized acrylic fibrous material derived from a fibrous precursorconsisting primarily of recurring acrylonitrile units, said preoxidizedacrylic fibrous material having a. a carbon content of up to about 65per cent by weight, b. a relatively amorphous X-ray diffraction pattern,c. a bound oxygen content of at least about 7 per cent by weight, and d.an absence of structural integrity when heated for two seconds at afiber temperature of 1,900* C., comprising: passing said preoxidizedacrylic fibrous material through a zone containing an organic protectiveagent capable of rendering said material resistant to loss of structuralintegrity when Heated for at least two seconds at a fiber temperature of1,900* C. selected from the group consisting of organic phosphoruscompounds, organic antimony compounds, organic tin compounds,halogenated alcohols, halogenated esters, halogenated organic acids,halogenated organic anhydrides, halogenated phenolic compounds,halogenated aliphatic hydrocarbons, halogenated aromatic hydrocarbons,halogenated aromatic ethers, silicone oils, and mixtures of theforegoing, wherein said preoxidized acrylic fibrous material isimpregnated with said organic protective agent, and passing saidimpregnated fibrous material through the luminous portion of a reducingflame generated by an acetylene-oxygen mixture imparting to said fibrousmaterial a minimum temperature of at least 1,900* C. in which the ratioof oxygen to acetylene is less than the stoichiometric amount requiredto completely oxidize the acetylene for a residence time of about 2 to24 seconds while said fibrous material is under a tension at leastsufficient to prevent visible sagging.
 18. A continuous processaccording to claim 17 wherein said preoxidized acrylic fibrous materialis derived from an acrylonitrile homopolymer.
 19. A continuous processaccording to claim 17 wherein said preoxidized acrylic fibrous materialis derived from an acrylonitrile copolymer which contains at least about85 mol per cent of acrylonitrile units and up to about 15 mol per centof one or more monovinyl units copolymerized therewith.
 20. A continuousprocess according to claim 17 wherein said preoxidized acrylic fibrousmaterial is passed through a zone containing said organic protectiveagent dissolved in a solvent.
 21. A continuous process according toclaim 20 wherein said solvent is a halogenated hydrocarbon.
 22. Acontinuous process according to claim 21 wherein said solvent istrichloroethylene.
 23. A continuous process according to claim 17wherein the residence for said impregnated fibrous material in saidreducing flame is about 6 to 17 seconds.
 24. A continuous processaccording to claim 17 wherein said preoxidized acrylic fibrous materialis a yarn.